Printed circuit boards and semiconductor devices are mostly produced by photolithographic processes. However, attentions have been focused on a printable electronic device technology which involves preparing an ink formulation by dispersing a recently developed nanometer-order metal material in some medium, conducting patterning by various printing methods by using the ink formulation, and assembling a device.
This technique is called printed electronics (hereinafter abbreviated as PE). There is a possibility that this technique can be used to mass-produce electronic circuit patterns and semiconductor elements through roll-to-roll processes and thus this technique is expected to bring an on-demand suitability and economy resulting from streamlining of processes and resource saving. Thus, there is an anticipation that the technique may help develop low-cost production methods for display devices, light-emitting devices, IC tags (RFIDs), etc.
PE requires a conductive material ink, a semiconductor material ink, and an insulating material ink as the basic materials. In particular, a conductive ink that contains a metal colloid (metal nanoparticles) of a noble metal (gold, silver, platinum, or the like) and a copper is important. Silver colloids and inks containing silver colloids are leading the development due to their economy and ease of handling.
Silver nanoparticles (or simply referred to as nano silver) constituting a silver colloid has far higher specific surface areas than bulk silver and have a strong tendency to fuse together and decrease surface energy. As a result, the particles fuse to each other at a temperature far lower than the melting point of the bulk silver. This phenomenon is called a quantum size effect (Kubo effect) and the advantage of using silver nanoparticles as the conductive material lies here.
In general, nano materials tend to be expensive since they are produced through special processes, which has obstructed widespread use of the materials. A liquid-phase reduction method that does not require special equipment such as one used in vacuum processes is advantageous for producing silver colloids at low cost. A liquid phase reduction method is a method for obtaining a silver colloid by reducing a silver compound by causing the silver compound to react with a reducing agent in a solvent. In order to limit the size of the silver nanoparticles to be generated to a particular level and realize a stable metal colloid state during this process, a technique of conducting reduction in the presence of a compound called a “colloid protecting agent” has been disclosed. A colloid protecting agent is a polymer compound designed to include a functional group, such as a tertiary amino group, a quaternary ammonium group, a heterocycle having a basic nitrogen atom, a hydroxy group, or a carboxyl group, that can coordinate to a metal particle (for example, refer to PTL 1).
As discussed above, the diameter of silver nanoparticles that are expected to undergo a desirable low-temperature fusion phenomenon is 50 nm or less. As the diameter becomes smaller than this, the amount of the required polymer compound serving as a colloid protecting agent also increases with the specific surface area; thus, the amount of the colloid protecting agent remaining on the silver nanoparticles is increased and low-temperature firing property (the property that the specific resistivity obtained by firing a thin film formed of the silver colloid at 100° C. to 150° C. is on the 10−6 Ωcm order) becomes difficult to attain, which has been a problem. From the viewpoint of designing a conductive material, a colloid protecting agent needs to exhibit plural properties simultaneously: capacity to make small particles, capacity to protect and stabilize particles, and capacity to rapidly detach from the particle surfaces during sintering so as not to inhibit fusion between particles. It has been difficult to attain these properties simultaneously by only using a technique that uses a commercially available polymer pigment dispersant, such as Solsperse (Zeneca) or Flowlen (Kyoeisha Chemical Co., Ltd.), or a commercially available polymer compound having a pigment affinic group (amine) in the main chain/side chain and two or more solvent affinic portions, etc. (for example, refer to PTL 2 to PTL 4).
To address this problem, there has already been disclosed that a polyalkyleneimine-polyalkylene glycol-hydrophobic segment ternary polymer and a polyalkyleneimine-polyalkylene glycol binary polymer are effective for producing silver nanoparticles (for example, refer to PTL 5 to PTL 7). However, a product that strikes a good balance among control of particle size and distribution, stability of colloidal solutions, and practical electrical conductivity by low-temperature firing has not been obtained from the use of these compounds.